This invention relates to molding apparatus and more particularly to molding apparatus having water-cooled core and cavity assemblies that cooperate to form hollow molded parts. The apparatus is an improvement on the molding apparatus shown in U.S. Pat. No. 5,498,150 issued to me on Mar. 12, 1996 and incorporated herein by reference.
To decrease cycle time and to insure good molding of either metal casting or injection molded plastic parts, it is advantageous to provide coolant flow passages within a mold apparatus. When cooling fluid is passed through such passages during the molding process, it removes heat from the apparatus. Where a molding apparatus includes a mold cavity part that has a plurality of female cavities formed within it, and where those female cavities are configured to receive core parts that cooperate with the female cavities to form the surfaces of a molded part, apparatus for cooling has included a round cooling tube that is formed by the interior surface of the core part. An example of such cooling apparatus is set forth in U.S. Pat. No. 4,655,280.
One problem with such arrangements is that molded parts having an elongated configuration must be formed, in part, by a core that has a length to width ratio large enough to allow the core to deflect when impacted by material that is injected against the core under high pressure conditions. Since many materials are best shaped and densified under such high pressure conditions, one problem with prior art apparatus is how to provide a low cost, elongated core that will not deflect or warp under high temperature and high pressure operation.
Another problem with such molding apparatus is that the coolant flow is restricted to an annular surface in such a way as to limit the removal of heat from each of the mold cavities.
U.S.Pat. No. 5,498,150 issued to me on Mar. 12, 1996 addresses these problems by disclosing a mold assembly having a mold part that forms the outer wall of a molded part. The mold part has a plurality of elongated cavities within it. A fluid cooled core is inserted within each of the elongated cavities before injecting material into the mold assembly. Each fluid cooled core has an outer surface that forms an inner wall of a tubular portion of the molded part. Each fluid cooled core includes a plurality of circumferentially spaced, longitudinal ribs or fins that extend integrally and radially inward from around an inner circumferential surface of the core. The ribs reinforce each fluid cooled core radially inwardly of their respective elongated cavities and along the length of each fluid cooled core. This construction controls core deflection that results from high-pressure injection of a material to be molded. A separate fluid inlet pipe is disposed coaxially within each core and is radially spaced from the inner circumferential surface of each core. An outer end of each fluid inlet pipe is adapted to be connected to a source of coolant and an inner end is in communication with a plurality of circumferentially spaced passages defined by the ribs within each core. Each of these passages returns the flow of cooling fluid to an elongated annular passage that extends along the length of the core between the fluid inlet pipe and the inner surface. Radially inner edges of the ribs are configured to slidably receive an inner length of the fluid inlet pipe during core assembly.
After assembly, each fluid inlet pipe lies in contact with or in close proximity to the rib inner edges. While this design is more resistant to warping and deformation during injection than prior art assemblies, the fluid inlet pipe still has some freedom to rotate, flex, warp and otherwise deform under the force of pressurized injections of molten material during molding. In addition, while the temperature of the core can be controlled using the fluid cooling system, the cavity portion of the mold cannot.
In my copending U.S. patent. application Ser. No. 09/153,956 filed Sep. 16, 1998, incorporated herein by reference a further improvement is provided that defines a fluid cooled core that is stronger, more rigid and therefore more resistant to deflection and warping under high temperature and high-pressure operation and a configuration that is able to cool both inner and outer walls of a tubular portion of a molded part. A mold assembly is provided that includes spaced, moveable mold parts, one mold part including a hollow cavity having a cavity inner surface configured to form an outer wall of an elongated, generally annular molded part and the other mold part including a corresponding core element having a core outer surface configured to form an inner wall of the molded part. The core element is at least partially disposed within the cavity and the core outer surface is spaced from the cavity inner surface defining a generally annular mold chamber for receiving molten material to be molded to the shape of the molded part. The core element has an open end and a closed end that defines a hemispherical portion of the core outer surface. The core element further includes a generally tubular core inner surface and a plurality of circumferentially spaced longitudinal ribs that integrally extend radially inward from the core inner surface. The ribs define a plurality of elongated coolant passages. A fluid inlet pipe is coaxially disposed within the core element. The fluid inlet pipe has a pipe outer surface spaced from the core inner surface, one end of the fluid inlet pipe configured to connect to a source of coolant and a second end of the pipe being in fluid communication with the plurality of coolant passages. In this arrangement greater strength and the heat transfer is obtained since the ribs are integral to both the fluid inlet and the core element to reinforce the core element radially inwardly of the mold chamber and along the length of the core element against core deflection that results from high pressure injection of a material to be molded. This design is more resistant to warping and deformation during injection than prior art designs.
While suitable for their intended purpose such arrangements have used stainless steel rather than copper or other softer materials. Copper core material, while having the ability to improve heat transfer from the core to the coolant fluid can be damaged by impacts thereon during the molding process.
In accordance with this invention a mold assembly is provided that includes spaced, moveable mold parts, one mold part including a hollow cavity having a cavity inner surface configured to form an outer wall of an elongated, generally annular molded part and the other mold part including a corresponding core element having a core outer surface configured to form an inner wall of the molded part. The core element is at least partially disposed within the cavity and the core outer surface is spaced from the cavity inner surface defining a generally annular mold chamber for receiving molten material to be molded to the shape of the molded part. The core element has an open end and a closed end that defines an end portion of the core outer surface. The core element further includes a first portion of a high strength material defining a generally tubular core inner surface; a plurality of circumferentially spaced longitudinal ribs can be provided that extend radially inward from the core inner surface through part of the length of the core element. The ribs define a plurality of elongated coolant passages that receive return coolant flow from an annular space formed between a second reduced diameter hollow tip portion of the core element. A fluid inlet pipe is coaxially disposed within the core element. The fluid inlet pipe has a pipe outer surface spaced from the core inner surface, one end of the fluid inlet pipe connects to a source of coolant and a second end of the pipe is in fluid communication with the plurality of coolant passages. Such a construction is improved by providing a bimetal core element having a first portion formed of relatively high strength lower heat transfer material such as stainless steel and having a second portion defining a core element tip formed of a high strength alloy material to resist impact damage while providing increased thermal diffusivity and increased thermal conductivity. A fluid inlet pipe is assembled axially within the bimetal core element. Furthermore, the first and second portions can be connected by suitable bonding techniques so that the preexisting core elements of the types shown in copending United States Patent Applications can be upgraded.
The method comprises the steps of cutting the end of a core; providing a hollow tip of higher thermal conductivity than the cut end; placing the hollow tip over the cut end and thereafter bonding an end of the hollow tip to the cut end of the core.